1. Field
The present invention relates to the field of analog-to-digital conversion.
2. Description of the Related Art
The general technique of providing analog-to-digital (A/D) or digital-to-analog (D/A) conversion of signals is well known in the art. Generally, the sampling rate that is required to sample an analog signal for A/D conversion must be twice the highest frequency component being sampled. This rate is known as the Nyquist rate. More recently, oversampling methods have been utilized for A/D and D/A conversion. In an oversampling type of convertor, the sampling rate is much higher than the Nyquist rate.
An advantage of using the oversampling technique is in the precision provided by the converter. With converters operating under the Nyquist rate for sampling, a certain amount of precision is required for the conversion. For example, in converting an analog signal into a 16-bit digital format, 16-bit precision is required. Accordingly, circuits will need to be designed having components which will meet this precision. In many instances, closely trimmed circuit components or precision matching (or compensating) circuits are required to meet the precision.
However, when sampling at a rate much higher than the required Nyquist sampling rate, the oversampling technique permits circuit precision to be relaxed significantly. For example, an A/D oversampling converter implements an oversampling modulator, the modulator output can be a single bit output. The circuit precision needs only to meet this 1-bit output. Accordingly, closely trimmed circuit components are generally not needed. Additionally, 1-bit precision can be readily met by current generating CMOS (complementary-metal-oxide-semiconductor) components.
The disadvantage of using the oversampling technique is the added requirement that the output generally needs to be reduced at the eventual output of the converter. That is, the higher sampling rate is usually returned to the Nyquist rate. In the above example, a multiple number of the 1-bit outputs will need to be combined to form a single output (e.g., 16-bit, 32-bit, etc.). However, the oversampling technique is preferred in many applications, since the cost savings in using less precise circuit components outweigh the additional digital signal processing needed at the back end of the converter.
One well-known type of oversampling A/D conversion uses a modulator commonly referred to as a delta-sigma modulator. Delta-sigma modulation is a method for encoding high-resolution signals into lower-resolution signals using pulse-density modulation. In an A/D converter (ADC) using a delta-sigma modulator, an integrator and a comparator are utilized at the front end of the converter to provide the quantization of the analog signal. Then, a low-pass filter and a decimator are utilized for digital signal processing to provide a corresponding digital signal at the Nyquist rate. However, the circuit precision of the analog circuitry can be relaxed, due to the use of the higher sampling rate.
When delta-gamma modulators are utilized, the modulator can be designed for higher than the first order of operation. Higher order operation of a delta-sigma modulator is desirable, since lower sampling rates can be utilized to obtain the same precision as operating the modulator at a lower order but with higher sampling rates. However, at higher order operation, stability is a concern. That is, the non-linear response of the delta-sigma comparator in the feedback path causes an unstable behavior.
It is noted that the instability condition is different than an overload condition. In an overload condition, the modulator experiences a degraded signal-to-noise ratio when the input amplitude exceeds a certain value, but the modulator can recover when the overload condition is removed. Instability is also a function of the amplitude of the input signal, but in this instance (unlike the overload condition), the modulator cannot recover from an unstable behavior with the reduction of the input signal. In order to return the system to its proper operating behavior, the state variables of the modulator can be reset to values within a stable state space. Resetting the values to a zero condition will generally suffice.